Vias are electrically conductive paths which are used to connect operably adjoining layers of multilayer circuits. The conventional method for producing vias in multilayer circuits is to screen print a pattern of dielectric paste over an underlying conductive pattern by which via holes are formed by the opaque areas of the screen. The printed dielectric layer is then fired and the via holes are filled by screen printing with a conductive thick film paste. Another method for making vias is to punch holes in a dielectric green (unfired) tape, fill the via holes by screen printing in the above-described manner, and fire the tape to remove the organic medium. These methods for making vias for multilayer circuits are, however, limited in that evenly shaped, symmetrical vias can be made reliably only if they are 8-10 mils in diameter or larger for thick film and 4 mils in diameter or larger for green tape. When it is attempted to make smaller vias by these methods, the via holes tend to fill in during printing and firing.
In an attempt to improve upon the conventional screen printing method, various experimenters have suggested the use of laser drilling to form the vias. For example, Cocca et al. have suggested the laser drilling of unfired thick film dielectrics overlying thick film conductors (Cocca et al, Laser Drilling of Vias in Dielectric for High Density Multilayer Thick Film Circuits, Solid State Technology, September 1978, p 63-66). In drilling via holes by this method, Cocca et al used a pulsed YAG laser at a power level of 42-47 mj using a single laser pulse, which resulted in the removal of the gold conductor pattern underlying the laser-drilled via hole. In addition, Barrett et al. disclosed the laser drilling of fired dielectric layers using a pulsed YAG laser set at 1 W having a Q-rate of 1 kHz. The study showed that the depth of the drilled holes was related directly to the number of pulses at a given power level (Barret et al., A Method for Scanning Electron Microscope Analysis of Laser Drilled Thick Film Vias, Hybrid Circuits, No. 4, Spring 1984, p 61-63).
More recently, Kawasaki et al, in copending patent application S.N. PCT/US 90/06160, filed Oct. 25, 1990, disclosed the use of a laser beam to form via holes in an inorganic insulating (dielectric) layer having a transparent protective layer on a beam-reflective substrate, filling the via holes with thick film paste, removing the protective layer, and then repeating the sequence to obtain the desired number of layers, after which the entire assemblage is cofired.
In addition, Wang in pending U.S. patent application Ser. No. 07/739.205 filed Aug. 1, 1991 disclosed a method for drilling via holes in unfired dielectric layers using a YAG laser under carefully controlled operating conditions including a pulse rate of 1 kHz. The pulsed laser beam is positioned by means of a galvonometric beam positioner to facilitate rapid drilling sequence of single holes.
Each of the prior art methods for laser drilling via holes uses either a YAG or CO.sub.2 laser or relatively long wave length lasers, which rely upon the application of intense heat to effect almost explosive ablation of the organic materials. Such methods are suitable for many applications. However, they are less desirable for drilling very small vias because the resolution of the vias is less than desired. Fine via resolution is particularly important when the multilayer device contains dense patterns of vias, e.g. 20 vias/cm.sup.2 or greater.